The operating principle of an inductive sensor is based upon the variation of coupling between a primary winding and the secondary windings of a transformer operating at high frequency, without the use of a magnetic circuit. The coupling between these windings varies according to the position of a movable (electrically) conductive component, generally described as a “target”. Currents induced in the target modify the currents induced in the secondary windings. By the adaptation of the configuration of the windings, and knowing the current injected into the primary winding, measurement of the current induced in the secondary windings permit the determination of the position of the target.
Document EP0182085, incorporated herein by reference, describes the principle of an inductive sensor of this type.
For the integration of an inductive sensor of this type in a device, specifically an electronic device, the configuration of the aforementioned transformer on a printed circuit board is known. The primary winding and the secondary windings are thus constituted as tracks on the printed circuit board. The primary winding is then, for example, supplied by an external high-frequency power source, and the secondary windings are the site of currents induced by the magnetic field which is generated by the flow of current in the primary winding. The target, which is a conductive part, for example of metal construction, can assume a simple shape. For example, this part can comprise a cut-out from a metal sheet. For the execution of a linear sensor, the cut-out for the formation of the target may be, for example, of rectangular shape whereas, for a rotary sensor, said cut-out will assume, for example, the shape of an angular sector of radius and angle which are appropriate to the movement of the component.
Generally, two series of secondary windings are configured for the execution, over the full length of travel of the sensor, of sine and cosine functions for the position of the target. Such functions (sine and cosine) are well-known, and can easily be processed by an electronic system. By establishing the ratio of the sine to the cosine, then applying an arctangent function, an image of the position of the target is obtained. The argument of the sine and cosine functions is a linear (or affine) function of the position of the target, the travel of which thus represents a proportion, of varying magnitude, of the spatial period of these trigonometric functions.
In physical terms, modification of the coupling between the primary circuit and the secondary circuits is achieved by means of the electromagnetic skin effect, which will be familiar to a person skilled in the art. As the primary circuit is supplied by a high-frequency source, phenomena occurring throughout the sensor will be high-frequency phenomena. The target, the position of which is to be identified, is a solid conductive part, and is the site of substantial induced currents. The depth of penetration of these induced currents is relatively low (hence the name “skin effect”). For example, this depth is of the order of 50 μm, in the case of an aluminum target. Consequently, induction does not penetrate the target, and the magnetic flux generated by the primary circuit thus bypasses the target. As a result, the field lines are substantially modified. This modification is perceived by the secondary circuits which, depending upon the position of the target, receive an increased or reduced flux. These fluxes, which vary as a function of the position of the target, are also variable as a function of time, and thus generate a voltage at the terminals of the secondary circuits. By the measurement of these voltages, a signal is obtained which, further to analysis, permits the identification of the position of the target.
Where it is not possible to arrange a sensor at the end of a shaft, in order to determine the angular position of said shaft, the configuration of the shaft with a helix, which is arranged opposite a linear sensor, is known. In practice, where a helix is considered in rotation in relation to a fixed point, a surface in axial motion will be seen from said fixed point. Accordingly, the entire process proceeds as if the target were in linear motion, in opposition to the sensor.
A linear sensor can therefore provide an indication of the angular position of a shaft, by adapting the form of the target. However, where the shaft, the angular position of which is to be identified, moves axially, even in the event of stray movements, the angular measurement is distorted as a result of this angular movement.